Github user wuchong commented on the issue:
https://github.com/apache/flink/pull/2653
Because SQL doesn't have standard user-defined table function, we introduce
MS SQL Server's `CROSS/OUTER APPLY` to support UDTF , see
https://technet.microsoft.com/en-us/library/ms175156(v=sql.105).aspx .
NOTE: In order to implement the `CROSS/OUTER APPLY`, we need to modify
Calcite's SQL Parser. So we include `Parser.jj` from Calcite source code and
add some modifications (search FLINK in this file).
See
[this](https://docs.google.com/document/d/15iVc1781dxYWm3loVQlESYvMAxEzbbuVFPZWBYuY1Ek/edit#)
for more UDTF design,
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
